Phantom based MR field mapping of the polarizing magnetic field

ABSTRACT

The present invention provides a phantom ( 200 ) for use in a magnetic resonance (MR) imaging system ( 110 ) with a set of resonating volumes ( 206 ) positioned in a base body ( 202 ), whereby the base body ( 202 ) has a spherical or ellipsoid shape in accordance with a volume of interest ( 203 ) of the MR imaging system ( 110 ), and the resonating volumes ( 206 ) are located at a circumference of the base body ( 202 ). The phantom is used in a method for evaluating the magnetic field of a main magnet ( 114 ) of a magnetic resonance (MR) imaging system ( 110 ), comprising the steps of positioning the phantom ( 200 ) within the main magnet ( 114 ), performing a 3D spectroscopic MR measurement of the phantom ( 200 ) using the MR imaging system ( 110 ), thereby measuring resonances of the resonating volumes ( 206 ), assigning the measured resonances to the resonating volumes ( 206 ), and evaluating the magnetic field of the main magnet ( 114 ) from the MR measurement of the phantom ( 200 ) based on the measured resonances of the resonating volumes ( 206 ). Accordingly, the MR imaging system itself is directly used for determining the magnetic field of its main magnet. Accordingly, the MR imaging system itself can be used as measurement equipment, instead of a separate NMR magnetometer, which is required for conventional determination of the magnetic field.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/IB2013/060809, filed on Dec.11, 2013, which claims the benefit of U.S. Patent Application No.61/738489, filed on Dec. 18, 2012. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to the field of magnetic resonance (MR) imagingsystems, and in particular to the area of MR magnetic field mapping forMR imaging systems.

BACKGROUND OF THE INVENTION

In magnetic resonance (MR) imaging systems, a main magnet is used togenerate a strong static magnetic field. In order to perform MRmeasurements with a good accuracy, it is required that the staticmagnetic field is homogenous in a volume of interest. The volume ofinterest corresponds to an examination space of the MR imaging systemand is typically a spherical or ellipsoid space with a diameter of about50 centimeters. A variation of the static magnetic field within thevolume of interest of less than 20 ppm is generally required. Beforefield homogeneity correction (otherwise known as shimming), a typicalmain magnet can have an inhomogeneity of about 500 ppm. Adjustment ofthe magnetic field is required, e.g. by adding magnetic material withinthe main magnet or by setting appropriate currents in adjustment coils.Before such corrections can be made, an accurate measurement of themagnetic field inside the magnet is needed.

Determining the static magnetic field inside the volume of interestwhich is usually a sphere or spheroid space located in the centre of themagnet, is also referred to as field mapping. This field mapping of anMR imaging system involves accurate determination of the magnetic fieldin a large number of locations. In known methods for determining thestatic magnetic field, the field is sampled over a closed surfacebounding the volume of interest; if the field on the surface of thevolume of interest is known, it can be reconstructed inside the entirevolume enclosed by this surface. Measurements of the magnetic field areperformed in 12-24 concentric circles about the longitudinal axis of themain magnet, also referred to as z-axis. Each circle is provided in aplane rectangular to the z-axis, and the measurements are provided in anangular distance of 15-30 degrees around the z-axis.

Conventional measurement methods employ a Nuclear Magnetic Resonance(NMR) magnetometer or an array of such magnetometers. The NMRmagnetometer is moved within the static magnetic field of the mainmagnet to desired sample locations in order to perform the requiredmeasurements as specified above. The movement is realized by use of aholding apparatus, which is usually mechanically operated in order toreduce influences on the static magnetic field.

The NMR magnetometer and the holding apparatus to move the NMRmagnetometer(s) are complicated to handle and expensive. The method formapping the MR magnetic field using the NMR magnetometer and the holdingapparatus is time consuming and also difficult to execute. Accordingly,an improvement is desired.

An additional problem arises since the static magnetic field isinfluenced by the location where the MR imaging (MM) system containingthe main magnet is located. Therefore, the homogeneity of the mainmagnet has to be verified every time the MRI system is moved, inparticular when the system is installed at a new location. Accordingly,availability of the NMR magnetometer and the holding apparatus arereduced by way of transport time, and there is an increased risk ofdamaging or loss of the NMR magnetometer and the holding apparatusduring transport. The same problem arises each time maintenance of themain magnet has to be performed, i.e. each time the static magneticfield is calibrated.

SUMMARY OF THE INVENTION

It is an object of the invention to facilitate the mapping of a mainmagnet of a magnetic resonance (MR) imaging system.

In one aspect of the present invention, the object is achieved by amethod for evaluating the magnetic field of a main magnet of a magneticresonance (MR) imaging system, comprising the steps of providing aphantom with a set of resonating volumes positioned in a base body,whereby the base body has a spherical or ellipsoid shape in accordancewith a volume of interest of the MR imaging system, and the resonatingvolumes are located at a circumference of the base body, positioning thephantom within the main magnet, performing a 3D spectroscopic MRmeasurement of the phantom using the MR imaging system, therebymeasuring resonances of the resonating volumes, assigning the measuredresonances to the resonating volumes, and evaluating the magnetic fieldof the main magnet from the MR measurement of the phantom based on themeasured resonances of the resonating volumes.

The 3D spectroscopic MR measurement refers to a measurement of preciseresonance frequencies at each measurement point. The 3D spectroscopic MRmeasurement is performed such that each of the individual resonantvolumes can be identified and such that the NMR resonance frequency ofeach of the resonance volumes is obtained. The particular frequency of amagnetic resonance of a resonating volume indicates the strength of themagnetic field at the location of the resonating volume. With themagnetic field known at the circumference of the volume of interest, thestatic magnetic field within the entire volume of interest can be fullydetermined. All spatial information is obtained by using phase encodinggradients. Using a measurement sequence with only phase-encodinggradients, the geometric distortion of the measurement is onlydetermined by gradient non-linearity. In each individual measurement,also referred to as phase encoding step, a point in the 3D acquisitionspace, also referred to as k-space, is acquired. From the 3D k-spacedata, the signals in the 3D spatial domain can be mathematicallyreconstructed. According to a preferred embodiment, the data are sampledon a regular 3D grid, allowing signal reconstruction on a regular 3Dgrid in the spatial domain by a Fast Fourier Transform.

The set of measurements is preferably processed by a computer such as toproduce a table, assigning a measured field value to each of thelocations of the resonant volumes. This field map can then be furtherprocessed to analyze the characteristics of the field inside the volumeof interest and to determine corrective actions required to make thefield of the magnet homogeneous.

In order to speed up the spectroscopic MR measurement, a resolution canbe chosen in the x, y and z-direction of the MR imaging system to besufficient to identify the resonating volumes. A preferred resolutionsuitable for the spectroscopic MR measurement contains 80 to 200measurement samples per axis in the x/y-plane and 1 to 30 samples in thez-direction. Further preferred, about 120×120 individual measurementsamples are taken in the x/y-plane and about 10 samples are taken alongthe z-axis. With the known structure of the phantom, i.e. the knownpositions of the resonating volumes, the spectroscopic measurement canbe performed and the resonating volumes can be matched to measuredresonances in the 3D spectroscopic MR measurement. Preferably, the exactposition is derived from the known structure of the phantom.Accordingly, an accurate placement of the phantom in the volume ofinterest is required. Using this phantom, the MR imaging system can beused directly for determining the magnetic field of its main magnet.Therefore, the MR imaging system itself can be used as measurementequipment, instead of a separate NMR magnetometer, which is required forconventional determination of the magnetic field. This measurement ismuch cheaper and more reliable than a measurement with a dedicatedmagnetometer system. Furthermore, the measurement results can bedirectly used for a calibration of the MR imaging system, if themeasurement is performed by the MR imaging system itself. An accurateplacement of the phantom with the planes having the requiredx/y-orientation is required. Typically, an accuracy of 2 to 3 mm isrequired for each degree of freedom, i.e. the x, y, z-axis and the threerotational axis.

The base body of the provided phantom is preferably made of plastics,e.g. of polycarbonate. The base body can have any suitable structure.Preferably, it is provided as an essentially hollow body. Alternatively,resonating volumes can be interconnected within the base body, with theresonating volumes defining the shape of the base body. In analternative embodiment, the base body is made of anothernon-electrically conducting material with a low magnetic susceptibility,

The resonating volumes are provided within the base body. Preferably,the resonating volumes are provided by enclosures of a resonating mediumwithin the base body. The resonating medium is a medium generating amagnetic resonance when subjected to the appropriate combination of astatic magnetic field and a RF field. The resonating volumes have anysize and shape suitable to be easily detected as separate volumes.Preferably, the resonating volumes have a spherical shape with adiameter of less than one centimeter, further preferred with a diameterof two to three millimeters. The base body can be provided with bores,which are filled with the resonating medium and sealed afterwards.Preferably, the resonating medium is water. Since the resonating volumesare provided at the circumference of the base body, a magnetic field atthis circumference can be evaluated.

The circumferential magnetic field is suitable to determine the magneticfield within the entire phantom, i.e. the volume enclosed by the set ofsample volumes. Accordingly, only a low number of resonating volumes isrequired. Resonating volumes within the entire circumference of thephantom are not required. Preferably, the resonating volumes are evenlydistributed over the circumference of the base body and have the samevolume. A typical volume of interest within a whole-body MR imagingsystem has a spherical shape with a diameter of about 50 cm. For thistypical volume of interest it is preferred to provide the phantom withat least 100 resonating volumes. Further preferred, the number ofresonating volumes is at least 200. This enables a sufficient mapping ofthe magnetic field without excessive measuring effort, since themeasuring effort increases with the number of resonating volumes.

According to a preferred embodiment the step of assigning the measuredresonances to the resonating volumes comprises identifying the measuredresonances in the spatial domain. Hence, the resonating volume can beidentified based on the position where it appears in the 3D image. Theposition is characterized by an angular and a radial position and adistance from the z=0 plane. In particular, the effect of backfoldingcan be used to perform combined measurements for resonating volumes indifferent locations in the longitudinal direction of the MR imagingsystem, i.e. in the z-axis. Due to the backfolding effect, measurementsof resonating volumes of different z-positions can be seen in onemeasurement of the z-axis. This method is in contrast to conventionalmeasurements, where the volume of the phantom is fully within the 3Dscan volume and the number of phase encoding steps is large enough toresolve all resonant volumes, thus avoiding backfolding. Due to theknown positions of the resonating volumes of the phantom, the number ofmeasurements in the z-axis can be reduced to speed up the measurement.Nevertheless, the frequency for all resonating volumes can be obtainedfrom the measurements and the magnetic field can be fully determinedbased on a reduced number of phase encoding steps in the z-axis and theselected size of the 3D imaging volume, which is preferably smaller thanthe extent of the phantom in z-direction. Further preferred, the phantomis provided with the resonating volumes arranged in parallel planes, sothat all resonating volumes can be measured with a minimum number ofphase encoding steps in z-direction. Further preferred, the volumes canbe distinguished due to their position within the phantom, i.e. arotational angle of the resonating volumes of a ring compared to theresonating volumes of different ring.

According to a preferred embodiment the step of assigning the measuredresonances to the resonating volumes comprises generating a catch zonefor each resonating volume within the 3D spectroscopic MR measurement.The frequency of the spectroscopic measurement within this catch zone isthen used as spectroscopic measurement from the resonating volumecorresponding to the catch zone. This allows a reduction of theresolution of the spectroscopic MR measurement, and the time fordetermining the static magnetic field can be reduced. Depending on thekind of used phantom, two- or three-dimensional catch zones can bedefined for each resonating volume. Preferably, the catch zone is atwo-dimensional zone, further preferred a zone in the x/y-plane of theMR imaging system. When the resonating volumes are positioned inx/y-planes, the z-position of the catch zone is predefined, so thatcatching along the z-axis is not required. The distances in thez-direction are sufficiently large to distinguish resonating volumes ofthe different planes.

According to a preferred embodiment the step of providing a phantomcomprises providing the phantom with the resonating volumes beingarranged in different planes, which are arranged in parallel to eachother. Accordingly, the resonating volumes are arranged in circular orelliptic rings. The arrangement in planes facilitates the manufacturingof the phantom and the placement of the phantom within the volume ofinterest. The arrangement can be realized by placing the resonatingvolumes of each plane in a circular, elliptic, or annular structure,whereby the structures of all planes are connected together to form thephantom. The planes can be provided with a constant distance betweeneach pair of planes, or the distance can be different for differentpairs of planes. Further preferred, the locations of the ringscorrespond to the foot-points of an n^(th) order Gaussian integration inθ direction, where θ is the spherical angular coordinate in the z-rplane and n is the number of rings. These angles are the zero points ofthe Legendre polynomial of order n. Preferably, the structure is a ringhaving a homogenous cross section in its circumferential direction suchthat all resonating volumes have the same susceptibility-relatedfrequency shift. Further preferred, all resonating volumes are providedin ring structures having the same cross section. A preferred phantomhas 20 to 30 planes, further preferred 24 planes. Preferably, each planehas a maximum of 20 to 30 resonating volumes. Since the center region ofthe phantom has a bigger diameter than its border region, it ispreferred that the planes of the center region are provided with ahigher number of resonating volumes compared to the border regions. Inthe method, the step of placing the phantom in the MR scanner comprisesaligning the phantom, such that the rings are approximately coaxial withthe longitudinal axis of the main magnet and the symmetry planeperpendicular to the axis or rotational symmetry of the phantomapproximately coincides with the corresponding symmetry plane of themain magnet.

According to a preferred embodiment the step of providing a phantomcomprises providing the phantom with the resonating volumes of eachplane being arranged with a uniform angular distance to each other. Thisdistribution of the resonating volumes enables the field mapping with ahigh accuracy at minimum computational effort for processing themeasurements. In case the planes are provided with different numbers ofresonating volumes, the angular distance can be different for thedifferent planes.

According to a preferred embodiment the step of providing a phantomcomprises providing the phantom with the resonating volumes of differentplanes being arranged at different angular positions. The angularpositions refer to a rotation of the resonating volumes of differentplanes relative to each other or in relation to a common coordinatesystem. This results in the resonating volumes of the respective planesbeing easily identifiable with the MR imaging system when performing aspectroscopic MR measurement. In general, an absolute angular positionof the resonating volumes is not important. The difference of theangular positions depends on the diameter of the phantom and the numberof resonating volumes distributed in a plane. Preferably, respectiveplanes have an angular difference which is a half of the angle betweenresonating volumes of one plane. Accordingly, the resonating volumes ofdifferent planes can be identified by their angular positions, which areknown due to the known phantom, even when the radial distance from thelongitudinal axis of the phantom is identical. Accordingly, whenspectroscopic measurements of resonating volumes cover resonatingvolumes of different rings due to backfolding, the resonating volumescan be reliably identified. The effect of backfolding, which is usuallynot desired, can be used with this phantom to reduce the number ofspectroscopic measurements in the longitudinal axis of the phantom,which corresponds to the longitudinal axis, i.e. the z-axis, of the MRimaging system when placed therein. The planes for applying therotational difference are preferably chosen among planes that show up ina single spectroscopic measurement. Accordingly, depending on theparameters of the measurement and the size of the phantom, the differentangular positions may be applied to different planes over the phantom.Further preferred, the angular positions vary for multiple planestogether.

According to a preferred embodiment the step of providing a phantomcomprises providing the phantom with a distance between the parallelplanes in a center region of the phantom being bigger than the distancebetween the parallel planes at a border region of the phantom.Therefore, also the angular difference between resonating volumes can beequally distributed in planes including the longitudinal axis of thephantom, i.e. planes rectangular to the x/y-plane. In the borderregions, the diameter of the phantom in adjacent planes shows biggervariations than in its center region, so that the resonating volumes ofthe different planes can easily be distinguished from the resonatingvolumes of other planes, even in the presence of backfolding.

In another aspect of the present invention, the object is achieved by amagnetic resonance (MR) imaging system, comprising a main magnet forgenerating a static magnetic field, a magnetic gradient coil system forgenerating gradient magnetic fields superimposed to the static magneticfield, an examination space provided to position a subject of interestwithin, at least one radio frequency (RF) antenna device that isprovided for applying an RF field to the examination space to excitenuclei of the subject of interest, and a control unit for controllingthe operation of the at least one RF antenna device, whereby themagnetic resonance imaging system is configurable to perform threedimensional MR spectroscopic measurements, and the control unit isconfigurable to perform the above method. The MR imaging system can beused together with the above phantom to perform the above method fordetermining the static magnetic field of its main magnet. Thisfacilitates installation and maintenance of the MR imaging system.

In another aspect of the present invention, the object is achieved by afield mapping system for evaluating a magnetic field, in particular of aMR imaging system, comprising a phantom with a set of resonating volumespositioned in a base body, whereby the base body has a spherical orellipsoid shape in accordance with a volume of interest of the MRimaging system, and the resonating volumes are located at acircumference of the base body, and a control device for operating theMR imaging system, whereby the control device is adapted to perform theabove method using the magnetic resonance (MR) imaging system.

In another aspect of the present invention, the object is achieved by asoftware package for upgrading a magnetic resonance (MR) imaging system,whereby the software package contains instructions for controlling theMR imaging system according to the above method.

Alternatively, a MR imaging system can be initially provided to performthe above method. Accordingly, the software package can be part of theinitial software of the MR imaging system, in particular, the softwarepackage can be part of a control unit of the MR imaging system.

In one aspect of the present invention, the object is achieved by aphantom for use in a magnetic resonance (MR) imaging system with a setof resonating volumes positioned in a base body, whereby the base bodyhas a spherical or ellipsoid shape in accordance with a volume ofinterest of the MR imaging system, and the resonating volumes arelocated at a circumference of the base body and arranged in differentplanes, which are arranged in parallel to each other, whereby theresonating volumes of each plane are arranged with a uniform angulardistance to each other, and the resonating volumes of different planesare arranged at different angular positions.

Such a phantom can be provided with low costs and is easy to handle. Thephantom is stationary, i.e. it does not have any moving parts and itdoes not have to be moved, so that the risk of damage or malfunction islow compared to a movable apparatus. With the resonating volumes at thecircumference of the phantom, the phantom enables the determination ofthe magnetic field within the entire volume of interest. Thedetermination of the magnetic field is also referred to as fieldmapping.

The base body is preferably made of plastics, e.g. of polycarbonate. Thebase body can have any suitable structure. Preferably, it is provided asan essentially hollow body. Alternatively, resonating volumes can beinterconnected within the base body, with the resonating volumesdefining the shape of the base body. In an alternative embodiment, thebase body is made of another material, which is electricallynon-conducting and has a low magnetic susceptibility.

The resonating volumes are provided within the base body. Preferably,the resonating volumes are provided by enclosures of a resonating mediumwithin the base body. The resonating medium is a medium generating amagnetic resonance when subjected to the appropriate combination of astatic magnetic field and a RF field. The resonating volumes have anysize and shape suitable to be easily detected as separate volumes.Preferably, the resonating volumes have a spherical shape with adiameter of less than one centimeter, further preferred with a diameterof two to three millimeters. The base body can be provided with bores,which are filled with the resonating medium and sealed afterwards.Preferably, the resonating medium is water. Since the resonating volumesare provided at the circumference of the base body, a magnetic field atthis circumference can be evaluated.

The circumferential magnetic field is suitable to determine the magneticfield within the entire phantom, i.e. the volume enclosed by the set ofsample volumes. Accordingly, only a low number of resonating volumes isrequired. Resonating volumes within the entire circumference of thephantom are not required. Preferably, the resonating volumes are evenlydistributed over the circumference of the base body and have the samevolume. A typical volume of interest within a whole-body MR imagingsystem has a spherical shape with a diameter of about 50 cm. For thistypical volume of interest it is preferred to have the phantom with atleast 100 resonating volumes. Further preferred, the number ofresonating volumes is at least 200. This enables a sufficient mapping ofthe magnetic field without excessive measuring effort, since themeasuring effort increases with the number of resonating volumes.

Accordingly, the resonating volumes are arranged in circular or ellipticrings. The arrangement in planes facilitates the manufacturing of thephantom and the placement of the phantom within the volume of interest.The arrangement can be realized by placing the resonating volumes ofeach plane in a circular, elliptic, or annular structure, whereby thestructures of all planes are connected together to form the phantom. Theplanes can be provided with a constant distance between each pair ofplanes, or the distance can be different for different pairs of planes.Further preferred, the locations of the rings correspond to thefoot-points of an n^(th) order Gaussian integration in θ direction,where θ is the spherical angular coordinate in the z-r plane and n isthe number of rings. These angles are the zero points of the Legendrepolynomial of order n. Preferably, the structure is a ring having ahomogenous cross section in its circumferential direction such that allresonating volumes have the same susceptibility-related frequency shift.Further preferred, all resonating volumes are provided in ringstructures having the same cross section. A preferred phantom has 20 to30 planes, further preferred 24 planes. Preferably, each plane has amaximum of 20 to 30 resonating volumes. Since the center region of thephantom has a bigger diameter than its border region, it is preferredthat the planes of the center region are provided with a higher numberof resonating volumes compared to the border regions. In the method, thestep of placing the phantom in the MR scanner comprises aligning thephantom, such that the rings are approximately coaxial with thelongitudinal axis of the main magnet and the symmetry planeperpendicular to the axis or rotational symmetry of the phantomapproximately coincides with the corresponding symmetry plane of themain magnet.

This distribution of the resonating volumes with a uniform angulardistance to each other enables the field mapping with a high accuracy atminimum computational effort for processing the measurements. In casethe planes are provided with different numbers of resonating volumes,the angular distance can be different for the different planes.

The angular positions refer to a rotation of the resonating volumes ofdifferent planes relative to each other or in relation to a commoncoordinate system. This results in the resonating volumes of therespective planes being easily identifiable with the MR imaging systemwhen performing a spectroscopic MR measurement. In general, an absoluteangular position of the resonating volumes is not important. Thedifference of the angular positions depends on the diameter of thephantom and the number of resonating volumes distributed in a plane.Preferably, respective planes have an angular difference which is a halfof the angle between resonating volumes of one plane. The resonatingvolumes of different planes can be identified by their angularpositions, which are known due to the known phantom, even when theradial distance from the longitudinal axis of the phantom is identical.Accordingly, when spectroscopic measurements of resonating volumes coverresonating volumes of different rings due to backfolding, the resonatingvolumes can be reliably identified. The effect of backfolding, which isusually not desired, can be used with this phantom to reduce the numberof spectroscopic measurements in the longitudinal axis of the phantom,which corresponds to the longitudinal axis, i.e. the z-axis, of the MRimaging system when placed therein. With the different angular positionsof the resonating volumes of different planes, crosstalk of theresonating volumes of different planes can be reduced. The planes forapplying the rotational difference are preferably chosen among planesthat show up in a single spectroscopic measurement. Accordingly,depending on the parameters of the measurement and the size of thephantom, the different angular positions may be applied to differentplanes over the phantom. Further preferred, the angular positions varyfor multiple planes together.

The mapping of the field of the MRI magnet comprises following steps, asalready discussed before: the phantom in placed in the main magnet of anMR scanner. An MRI scan is then performed in such a way that each of theindividual resonant volumes can be identified and such that the NMRresonance frequency of each of the resonance volumes is obtained. Thepreferred imaging technique for this is a so-called 3D spectroscopicimaging sequence, where all spatial information is obtained by usingphase encoding gradients. The resonance frequency thus found for eachresonant volume is a measure for the field of the magnet at thatlocation. The set of measurements is processed by a computer such as toproduce a table, assigning a measured field value to each of thelocations of the resonant volumes. This field map can then be furtherprocessed to analyze the characteristics of the field inside the volumeof interest and to determine corrective actions required to make thefield of the magnet homogeneous.

According to a preferred embodiment the distance between the parallelplanes in a center region of the phantom is bigger than the distancebetween the parallel planes at a border region of the phantom.Therefore, also the angular difference between resonating volumes can beequally distributed in planes including the longitudinal axis of thephantom, i.e. planes rectangular to the x/y-plane.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter. Suchan embodiment does not necessarily represent the full scope of theinvention, however, and reference is made therefore to the claims andherein for interpreting the scope of the invention.

In the drawings:

FIG. 1 is a schematic illustration of a part of an embodiment of amagnetic resonance (MR) imaging system in accordance with the invention,

FIG. 2 is an illustration of a phantom having a spherical shape inaccordance with the invention,

FIG. 3 is a partial view of the phantom of FIG. 2 showing half ahemisphere of rings forming a base body thereof and with resonatingvolumes in the ring, and

FIG. 4 is an illustration showing a visualization of measurement resultsfrom resonating volumes of different rings taken by a single measurementin the z-axis.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a schematic illustration of a part of an embodiment of amagnetic resonance (MR) imaging system 110 comprising an MR scanner 112.The MR imaging system 110 includes a main magnet 114 provided forgenerating a static magnetic field. The main magnet 114 has a centralbore that provides an examination space 116 around a center axis 118 fora subject of interest 120, usually a patient, to be positioned within.In an alternative embodiment a different type of MR imaging systemproviding an examination region within a static magnetic field is used.Further, the MR imaging system 110 comprises a magnetic gradient coilsystem 122 provided for generating gradient magnetic fields superimposedto the static magnetic field. The magnetic gradient coil system 122 isconcentrically arranged within the bore of the main magnet 114, as knownin the art.

Further, the MR imaging system 110 includes a radio frequency (RF)antenna device 140 designed as a whole-body coil having a tubular body.The RF antenna device 140 is provided for applying an RF magnetic fieldto the examination space 116 during RF transmit phases to excite nucleiof the subject of interest 120. The RF antenna device 140 is alsoprovided to receive MR signal from the excited nuclei during RF receivephases. In a state of operation of the MR imaging system 110, RFtransmit phases and RF receive phases are taking place in a consecutivemanner. The RF antenna device 140 is arranged concentrically within thebore of the main magnet 114. As is known in the art, a cylindrical metalRF screen 124 is arranged concentrically between the magnetic gradientcoil system 122 and the RF antenna device 140.

Moreover, the MR imaging system 110 comprises an MR image reconstructionunit 130 provided for reconstructing MR images from the acquired MRsignals and an MR imaging system control unit 126 with a monitor unit128 provided to control functions of the MR scanner 112, as is commonlyknown in the art. Control lines 132 are installed between the MR imagingsystem control unit 126 and an RF transmitter unit 134 that is providedto feed RF power of an MR radio frequency to the RF antenna device 140via an RF switching unit 136 during the RF transmit phases. The RFswitching unit 136 in turn is also controlled by the MR imaging systemcontrol unit 126, and another control line 138 is installed between theMR imaging system control unit 126 and the RF switching unit 136 toserve that purpose. During RF receive phase, the RF switching unit 136directs the MR signals from the RF antenna device 140 to the MR imagereconstruction unit 130 after pre-amplification.

FIGS. 2 and 3 show a phantom 200 according to a preferred embodiment foruse in the MR imaging system 110. The phantom 200 comprises a base body202 having a spherical shape in accordance with a volume of interest 203of the MR imaging system 110. The volume of interest 203 is part of theexamination space 116. For a typical MR imaging system 110 the volume ofinterest has a diameter of about 50 to 60 centimeters, which is also thediameter of the phantom 200.

The base body 202 is an essentially hollow body, which is formed by aset of 24 circular rings 204, each of which having the same rectangularcross-section, as can be seen in FIG. 3. The rings 204 are made ofpolycarbonate and define a circumference of the base body 202. The rings204 have a common rotational axis 205, which is also referred to asz-axis.

Each ring 204 defines a plane, and the planes are located in parallel toeach other. The distance between the rings 204 in a center region 208 ofthe phantom 200 is bigger than the distance between the rings 204 in aborder region 210 of the phantom 200, and the rings 204 in the centerregion 208 have a bigger diameter than the rings 204 in the borderregion 210, thereby providing the spherical shape of the phantom 200.

Resonating volumes 206 are provided within the rings 204 as enclosuresof a resonating medium, which is water in this embodiment. The water isfilled into bores within the rings 204, and the bores are sealed with aplug after receiving the water. The bores are drilled with a tool with atip angle of 120 degrees, and the bottom face of the plug is alsodrilled with this tool, so that the enclosed water volumes each have anapproximately spherical shape with a diameter of 3 mm. The bores of eachring 204 are positioned with equal angular distances between each other.The rings 204 in the center region 208 have 24 resonating volumes 206,and the rings 204 in the border region 210 have 12 resonating volumes206. Overall, the resonating volumes 206 are evenly distributed over thecircumference of the base body 202, where the resonating volumes 206 ofadjacent rings 204 are arranged with the same angular difference inplanes including the z-axis of the phantom 200.

As can be seen in table 1, the resonating volumes 206 of different rings204 are arranged with different angular positions. Details regarding theangular positions will be explained further below.

Now will be described a method for evaluating the magnetic field of themain magnet 114 of the MR imaging system 110. The MR imaging system 110is capable of performing a 3D spectroscopic MR measurement.

The phantom 200 is provided and placed within the Volume of interest 203with its z-axis 205 aligned with the center axis 118 of the MR imagingsystem 110. Accordingly, the rings 204 of the base body 202 areaccurately positioned in planes in the x/y-direction of the MR imagingsystem 110.

Next, the MR imaging system 110 executes a 3D spectroscopic measurementwith three phase-encoding directions. The 3D spectroscopic MRmeasurement refers to a measurement of detailed resonance frequencies ateach measurement point. The particular frequency of a magnetic resonanceof the water of a resonating volume 206 indicates the strength of themagnetic field at the location of this resonating volume 206. Theresolution in the x/y-direction of the MR imaging system is chosen to be120×120 individual measurement points, requiring 120 phase encodingsteps in x and y directions. The number of phase encoding steps inz-direction is set to be 10, resulting in 10 image slices inz-direction. In the measurement, backfolding leads to superposition ofthe information of 2-4 rings 204 in one slice. A measurement sequencewith only phase-encoding gradients is used, so that the geometricdistortion of the measurement is only determined by gradientnon-linearity.

After performing the measurement, the measured resonances are assignedto the resonating volumes 206. Accordingly, the measured resonance ofeach resonating volume 206 is identified in at least one of theindividual measurements. Therefore, each resonating volume 206 isidentified based on the position of its measured signal in the 3D image,using catch zones within the spectroscopic MR measurement of the phantom200. The identified frequency of the spectroscopic measurement withinthis catch zone is then used as spectroscopic measurement from theresonating volume 206 corresponding to the catch zone. The catch zonesare defined based on the known positions of the resonating volumes 206and a known field profile of the gradient coil of the MR imaging system110 and the parameters of the scan sequence.

The phase encoding steps in z-direction are selected and performed toallow signal reconstruction for rings having the indexes 8 through 17,as shown in table 1. With the 10 phase-encoding steps in z-axis 205,reconstruction of these 10 rings 204 is possible. Each reconstructedslice is in the z-axis 205 approximately centered at the position of therespective single ring 204. Due to intra-slice cross talk, each slicealso contains the information of adjacent rings 204. For example, slice5 not only contains the image of ring 204 with index 12 but also of theneighboring rings 204 having indexes 11 and 13, as illustrated in FIG.4. FIG. 4 shows the result for one phase encoding step in z-directionwith multiple resonances referring to resonating volumes 206 ofdifferent rings 204.

Identification of the resonating volumes 206 based on the position ofits measured resonance is performed. As can be seen in FIG. 4, theposition is unique including an angular and a radial position. The imageof ring 204 with index 12 is distinguished from unwanted information ofrings 204 with indexes 11 and 13 due to their relative angular offset of7.5 degrees. The rings 204 located outside the area covered by the 3Dspectroscopic MR scan with the 10 phase encoding steps show up in the MRmeasurement due to backfolding. As a further example, the image of thering 204 with index 7 is superimposed on the image of the ring 204 withindex 17 in slice 10, as indicated in table 1. Although the rings 204with indexes 7 and 17 have nearly identical diameters, they aredistinguished by their angular offset of 7.5.

As shown in table 1, a discontinuity in the alternating angular positionscheme appears between the rings 204 with index 8 and index 7, sincethese two rings 204 have the same angular position. Since the image ofthe ring 204 with index 18 is superimposed on the image of ring 204 withindex 8 in slice 1, a relative angular offset of 7.5 degrees is wantedto distinguish these two rings 204. Hence, ring 204 with index 18 hasthe same angular position as ring 204 with index 17. As the separationbetween the rings 204 of the phantom 200 along the z-axis 205 becomessmaller on approaching the border regions 210 of the phantom 200, groupsof multiple adjacent rings 204 of the border region 210 appear togetherin the same slice. For example, rings 204 with indexes 4 and 5 bothappear in slice 8, superimposed on the image of the ring 204 with index15. Because of this superposition of image information, an angularoffset of 7.5 degrees between rings 204 with indexes 4 and 5 isprovided. The first three rings 204 with indexes 1 to 3 at the borderregion 210 of the phantom 200 appear in slice 7, superimposed on theimage of the ring 204 with index 14. The first three rings 204 withindexes 1 to 3 are distinguished by radius, so that the angular positionis freely chosen, since it is not required to separate these rings 204.The same applies for the final three rings 204 with indexes 22 to 24 atthe other border region 210 of the phantom 200.

TABLE 1 ring numbers of the phantom, the slices of the image in whichthey appear and the applied angular offsets ring plane angle 1 7 15 2 715 3 7 7.5 4 8 3.25 5 8 −3.25 6 9 3.25 7 10 −3.25 8 1 −3.25 9 2 3.25 103 −3.25 11 4 3.25 12 5 −3.25 13 6 3.25 14 7 −3.25 15 8 3.25 16 9 −3.2517 10 3.25 18 1 3.25 19 2 −3.25 20 3 3.25 21 3 −3.25 22 4 7.5 23 4 15 244 15

It will be obvious, that in other embodiments having other values forthe number of rings 204 of the phantom 200, the size of the volume ofinterest 203 and the number of phase encoding steps of the 3D image, theof angular positions of the rings 204 is chosen differently.

Finally, when the resonance frequencies of all resonating volumes 206are known, the magnetic field of the main magnet 114 is evaluated. Fromthe measured resonances of the resonating volumes 206, the magneticfield of the main magnet 114 at the locations of the resonating volumes206 is calculated. With the magnetic field known at the circumference ofthe volume of interest 203, the static magnetic field within the entirevolume of interest 203 is determined.

Using this phantom 200 with the described method, the MR imaging system110 is used directly for determining the magnetic field of its mainmagnet 116. Processing steps subsequent to the measurement of thespectroscopic 3D measurement are in one embodiment performed using aseparate control device, which is connected to the MR imaging system110. Accordingly, the phantom 200 and the control device form a fieldmapping system for evaluating a magnetic field of a magnetic resonance(MR) imaging system 110 is provided, whereby the control device isadapted to perform the above method using the magnetic resonance (MR)imaging system 110.

In an alternative embodiment, the method steps are controlled directlyby the control unit 126 of the MR imaging system 110, or by a separateunit dedicated to performing the method.

The method according to this embodiment is implemented in softwareexecuted within the control unit 126 or the control device. Inparticular, a software package for upgrading the MR imaging system 110is provided for executing the method in the control unit 126, wherebythe software package contains instructions for controlling the MRimaging system 110 according to the above method.

In an alternative embodiment, the software package is integral part of acontrol software of the control unit 126 of the MR imaging system 110.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims. In the claims,the word “comprising” does not exclude other elements or steps, and theindefinite article “a” or “an” does not exclude a plurality. The merefact that certain measures are recited in mutually different dependentclaims does not indicate that a combination of these measures cannot beused to advantage. Any reference signs in the claims should not beconstrued as limiting the scope.

REFERENCE SYMBOL LIST

-   110 magnetic resonance (MR) imaging system-   112 magnetic resonance (MR) scanner-   114 main magnet-   116 RF examination space-   118 center axis-   120 subject of interest-   122 magnetic gradient coil system-   124 RF screen-   126 MR imaging system control unit-   128 monitor unit-   130 MR image reconstruction unit-   132 control line-   134 RF transmitter unit-   136 RF switching unit-   138 control line-   140 radio frequency (RF) antenna device-   200 phantom-   202 base body-   203 volume of interest-   204 ring-   205 rotational axis, z-axis-   206 resonating volume-   208 center region-   210 border region

The invention claimed is:
 1. A method for evaluating a magnetic field ofa main magnet of a magnetic resonance (MR) imaging system, comprisingthe steps of providing a phantom with a set of resonating volumespositioned in a base body, wherein the base body has a spherical orellipsoidal shape and comprises a plurality of rings in accordance witha volume of interest of the MR imaging system, and the resonatingvolumes are located at a circumference of the base body, the resonatingvolumes having a spherical shape, positioning the phantom within themain magnet, performing a 3D spectroscopic MR measurement of the phantomin which all spatial information is encoded by phase encoding gradients,using the MR imaging system, thereby measuring resonances of theresonating volumes, assigning the measured resonances to the resonatingvolumes, and evaluating the magnetic field of the main magnet from theMR measurement of the phantom based on the measured resonances of theresonating volumes.
 2. The method of claim 1, wherein the assigning themeasured resonances to the resonating volumes comprises identifying themeasured resonances in a spatial domain.
 3. The method of claim 1,wherein the assigning the measured resonances to the resonating volumescomprises generating a catch zone for each resonating volume within the3D spectroscopic MR measurement.
 4. The method of claim 1, wherein theproviding a phantom comprises providing the phantom with the resonatingvolumes being arranged in different planes, which are arranged inparallel to each other.
 5. The method of claim 4, wherein the providinga phantom comprises providing the phantom with the set of resonatingvolumes of each plane being arranged with a uniform angular distance toeach other.
 6. The method of claim 4, wherein the providing a phantomcomprises providing the phantom with the set of resonating volumes ofdifferent planes being arranged at different angular positions.
 7. Themethod of claim 4, wherein the providing a phantom comprises providingthe phantom with a distance between the parallel planes in a centerregion of the phantom being bigger than the distance between theparallel planes at a border region of the phantom.
 8. A magneticresonance imaging system, comprising a main magnet for generating astatic magnetic field, a magnetic gradient coil system for generatinggradient magnetic fields superimposed to the static magnetic field, anexamination space provided to position a subject of interest within, atleast one radio frequency (RF) antenna device that is provided forapplying an RF field to the examination space to excite nuclei of thesubject of interest, and a control unit for controlling the operation ofthe at least one RF antenna device, wherein the magnetic resonance (MR)imaging system is configurable to perform three dimensional MRspectroscopic measurements, and the control unit is configurable toperform the method of claim
 1. 9. A field mapping system for evaluatinga magnetic field of a magnetic resonance imaging system, comprising aphantom with a set of resonating volumes positioned in a base body,wherein the base body has a spherical or ellipsoidal shape in accordancewith a volume of interest of the MR imaging system, and the resonatingvolumes are located at a circumference of the base body, a controldevice for operating magnetic resonance imaging system, wherein thecontrol device is adapted to perform the method of claim 1 using themagnetic resonance imaging system.
 10. The field mapping system ofpreceding claim 9, wherein the distance between the parallel planes in acenter region of the phantom is bigger than the distance between theparallel planes at a border region of the phantom.
 11. A softwarepackage for upgrading a magnetic resonance (MR) imaging system, whereinthe software package contains instructions for controlling the MRimaging system according to claim
 1. 12. A phantom for use in a magneticresonance (MR) imaging system with a set of resonating volumespositioned in a base body, the resonating volumes being located at acircumference of the base body and arranged in different planes, whichare arranged in parallel to each other, wherein the set of resonatingvolumes of each plane are arranged with a uniform angular distance toeach other, the base body having a spherical or ellipsoidal shape, andcomprising a plurality of rings in accordance with a volume of interestof the MR imaging system, and the set of resonating volumes of differentplanes are arranged at different angular positions, wherein all spatialinformation used to obtain a 3D spectroscopic MR measurement of thephantom is obtained using phase encoding gradients.
 13. The phantom ofclaim 12, wherein the distance between the parallel planes in a centerregion of the phantom is bigger than the distance between the parallelplanes at a border region of the phantom.
 14. The phantom of claim 12,wherein the different planes are arranged in parallel to each other. 15.The phantom of claim 14, wherein the resonating volumes of each plane isarranged with a uniform angular distance to each other.
 16. The methodof claim 1, wherein each of the plurality of rings has the samerectangular cross-section.
 17. The phantom of claim 12, wherein each ofthe plurality of rings has the same rectangular cross-section.
 18. Themethod of claim 4, wherein each of the planes is defined by a respectiveone of the plurality of rings.
 19. The phantom of claim 12, wherein eachof the planes is defined by a respective one of the plurality of rings.